Apparatus for use in a NMR system comprises a magnetic field generator (1) surrounding a bore (3), for generating a magnetic field in a working volume (9) located in the bore. A sample support (11) can be removably inserted into a first end of the bore (3) so as to locate a sample in the working region (9). A probe (4) carrying RF generating and receiving coils (6-8) can be removably inserted into the other, second end of the bore (3) so as to locate the RF coils adjacent the working volume. A set of shim coils (19) located in the bore (3) about the working volume (9) cooperate with the magnetic field generator (1) to create a magnetic field in the working volume of sufficient uniformity to perform a NMR experiment on a sample. The RF receiving and generating coils (6-8) are located in a reduced diameter section of the probe (4) at its leading end. At least some of the shim coils (19) are located on a support (23) surrounding the reduced diameter section of the probe.